The present invention relates to a developing device for use i an image forming apparatus such as a copying machine, printer, or the like which utilizes the principles of electrophotography.
Dry-type developers or developing powders used in electrophotographic image development are roughly classified into two component developers and single component developers.
The two component developers are a mixture of a carrier and a toner. Therefore, the electrophotographic process employing such a two component developer requires a toner density controlling device for keeping the toner and the carrier mixed at a constant ratio. Another problem of the two component developer is that it should be replaced periodically since the carrier is degraded in use. To eliminate the above drawbacks, there has recently been proposed a developing process for developing latent electrostatic images with a single component developer containing no carrier. Such a developing process is disclosed in Japanese Laid-Open Patent Publication No. 54 (1979)-43038, Japanese Laid-Open Patent Publication No. 56 (1981)-110963, and U.S. Pat. No. 4,083,326, for example. FIGS. 1, 2, and 3 of the accompanying drawings show the developing arrangements disclosed in these prior publications.
FIG. 1 schematically illustrates an image developing device shown in Japanese Laid-Open Patent Publication No. 54(1979)-43038. The device includes a developing roller 1, a hopper 3 housing the developing roller 1 and containing toner 2, a blade 4 attached to the hopper 3, and a photosensitive drum 5. The developing roller 1 is made of metal and has surface irregularities. The developing roller 1 is supplied with the toner 2 from the hopper 3. When the developing roller 1 is rotated in the direction of the arrow, the supplied toner 2 is charged to a given polarity and coated on the pheripheral surface of the developing roller 1 by the blade 4 slidably held against the developing roller 1. The charged toner is then transferred from the developing roller 1 to a latent electrostatic image on the photosensitive drum 5 when the image confronts the developing roller 1, for thereby developing the image into a visible toner image.
As shown in FIG. 2, the developing device disclosed in Japanese Laid-Open Patent Publication No. 58(1981)-110963 includes a photosensitive drum 6, a developing roller 7 of electrically conductive urethane foam held slidably against the photosensitive drum 6, an electrically conductive fur brush 8 held slidably against the developing roller 7, a power supply 9, a voltage regulator 10 for regulating voltages to be applied by the power supply 9 to the fur brush 8 and the developing roller 7, and a hopper 11 containing toner 12. The toner 12 supplied from the hopper 11 to the fur brush 8 is triboelectrically charged by the fur brush 8, and then attracted from the fur brush 8 so as to be coated on the developing roller 7 by the voltage applied by the power supply 9. Thereafter, the toner 12 is applied from the developing roller 7 to a latent electrostatic image on the photosensitive drum 6 to develop the image. If the desired toner density is not achieved on the developed image, then the voltage regulator 10 is operated to control the voltages impressed on the developing roller 7 and the fur brush 8.
The developer applicator apparatus disclosed in U.S. Pat. No. 4,083,326 is illustrated in FIG. 3 of the accompanying drawings. The developer applicator apparatus comprises a developing roller 13, a hopper 15 containing toner 14, a blade 16, a photosensitive sheet 17, electrically conductive fur brushes 18, 19 slidably contacting the developing roller 13, a first power supply 20 for applying a voltage to the fur brush 18, and a second power supply 21 for applying a voltage to the developing roller 13. The voltage applied by the second power supply 21 is of a magnitude greater than the voltage applied by the first power supply 20, but lower than the potential of a latent electrostatic image on the photosensitive sheet 17. The toner 14 that is triboelectrically charged by the fur brush 18 is supplied from the hopper 15 through the fur brush 18 to the developing roller 13 under the potential difference between the first and second power supplies 20, 21. Then, after the toner 14 is adjusted into a thin flat layer by the blade 16, it is applied to the latent electrostatic image on the photosensitive sheet 17 to develop the image. Thereafter, residual toner 14 on the developing roller 13 is scraped off the fur brush 19 to eliminate the developing hysteresis on the developing roller 13.
The conventional developing arrangements are however disadvantageous in that the developer or toner cannot be uniformly charged and cannot be forming into a layer of even thickness on the developing roller, resulting in difficulty in reproducing images of high quality.
More specifically, in the construction shown in FIG. 1, the toner particles in the surface toner layer on the developing roller 1 are triboelectrically charged in contact with the blade 4, but those below the surface toner layer which are not held in contact with the blade 4 are not triboelectrically charged. While the charged toner particles are being transferred from the developing roller 1 to the photosensitive drum 5 for image development, the uncharged toner particles tend to be scattered around, smearing the developing device and fogging the developed image due to toner deposits on the non-image area on the photosensitive drum 5.
With the developing system shown in FIG. 2, the charged toner on the fur brush 8 is attracted onto the developing roller 7 and coated thereon under the electric field between the fur brush 8 and the developing roller 7. The developing system of this design requires a means for uniformly supplying the toner from the hopper 11 to the fur brush 8 since irregular toner coating would take place on the developing roller 7 unless a constant quantity of toner were supplied to the fur brush 8. Even if such a uniform toner supply means is provided, however, coated toner irregularities will still be caused on the developing roller 7 because of density variations of the fur of the fur brush 8, resulting in uneven toner densities on the developed image.
The device shown in FIG. 3 can solve the problems of the arrangements of FIGS. 1 and 2 by uniformly coating the charged toner on the developing roller 13. Nevertheless, the arrangement of FIG. 3 is structurally complex as the two fur brushes 18, 19 are required, one for supplying the toner and one for scraping off the toner. The location in which the developing device can be placed is limited since the toner-containing hopper is disposed below the developing roller 13. In addition, because the fur brushes are caused to rotate in a direction opposite to the direction in which the developing roller 13 rotates at positions where they contact each other, the scraped-off toner is scattered around by the furs of the fur brushes 18, 19 as they spring back out of contact with the developing roller 13. This is problematic since the scattered toner tends to smear the interior of the electrophotographic copying machine or fog the developed image.